As an ink composition for ink jet recording there has heretofore been widely used a water-soluble liquid ink composition. However, recording on paper into which an ink can easily infiltrate is liable to "bleeding". Thus, the recording medium for this purpose is limited to coated paper. When an ink is applied onto an overhead projector (hereinafter referred to as "OHP") sheet, it cannot be easily dried. Thus, OHP sheet needs to be subjected to special treatment on the surface thereof. Thus, as an ink composition which can provide a good printing quality regardless of the quality of the paper used, there has been provided a hot-melt ink composition comprising as a material a wax which stays as solid at room temperatures. The hot-melt ink composition is adapted to a hot-melt ink jet recording process which comprises liquefying it by heating or the like, and then applying some energy to the liquid so that it is jetted onto a recording medium on which it is then cooled and solidified while being attached thereto to form a recorded dot.
A great advantage of the above described ink jet recording process is that the above described hot-melt ink stays as solid at room temperatures and thus does not stain hands or any other circumferential things when handled. Another great advantage is that the vaporization of the ink during melting can be minimized, making it possible to prevent the clogging of the nozzle. Further, since the ink composition which has been attached to the recording medium is immediately solidified, it does not bleed on the recording medium. Thus, Various recording media such as Japanese paper, drawing paper and postcard can be used in the above described ink jet recording process without any pretreatment. U.S. Pat. Nos. 4,391,369 and 4,484,948 disclose an ink composition which can provide a good printing quality regardless of the quality of the paper used. JP-A-6-107987 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") and JP-A-5-194897 discloses a hot-melt ink composition for ink jet recording which comprises glyceride incorporated therein to exhibit excellent light transmission properties.
On the other hand, it has been a common practice to use a pigment as an ink colorant for the purpose of improving the weathering resistance of printed matters. Such an ink composition is used as an ink for various printers such as laser printer, fused transfer type printer, liquid ink jet printer and solid ink jet printer.
For example, many reports have been made on pigment ink for use in solid ink jet printer as in JP-A-3-37278, JP-A-4-339871, JP-A-5-16343, JP-A-5-105832, JP-A-6-49400, JP-A-6-228479, JP-A-6-228480, JP-A-6-306319, JP-A-7-109432, JP-A-7-196968, JP-A-7-278477, JP-A-7-306319, JP-A-7-316479, JP-A-7-331141, and JP-A-8-295836.
Further, JP-A-61-159470 discloses a hot-melt ink comprising a vehicle containing a C.sub.18-24 alcohol, the vehicle having incorporated therein graphite pre-dispersed in an oil carrier compatible with the vehicle. Moreover, JP-W-A-5-506881 (The term as used herein means an "international patent application published in its Japanese national stage") reports a hot-melt ink comprising an alcohol having a molecular weight of from 500 to 1,000 incorporated in a vehicle with other materials.
However, the above described hot-melt ink compositions are disadvantageous in that the materials constituting these ink compositions can hardly maintain a good compatibility therebetween. Thus, these ink compositions are liable to separation of components in molten form particularly when they comprise as a colorant an organic pigment having an excellent weathering resistance incorporated therein. As well known, the sedimentation of particles dispersed in a liquid changes depending on the diameter of the particles, the viscosity of the dispersion medium and the sedimentation time. The higher the viscosity of the dispersing medium is, the more difficultly can be sedimented the particles. On the other hand, the lower the viscosity of the ink for use in printing by an ink jet printer is, the more favorable for use in higher speed operation and higher density printing and more suitable for high reliability printing is the ink. Thus, the two requirements stand opposite each other.
The hot-melt ink jet recording process using an organic pigment having an excellent weathering resistance as a colorant has more advantages than the ink jet recording process using a dye. Thus, this hot-melt ink jet recording process is expected to be applied not only to OA apparatus, general household printers, facsimiles, etc. but also to indoor poster, outdoor poster, large-sized sign-board, decoration on automobiles, decoration on elevator and printing on cloth. However, as previously mentioned, this hot-melt ink jet recording process is disadvantageous in that the ink is liable to separation of components and the two requirements, i.e., high reliability and high printing quality cannot be satisfied at the same time. This disadvantage causes a bottleneck in the way of practical use.